The present invention relates to a liquid crystal display apparatus of active matrix type.
There is known a liquid crystal display apparatus of a type that has a plurality of pixel electrodes arranged in a matrix form and displays images by controlling voltages to be applied to the pixel electrodes through a plurality of switching elements connected to a plurality of scanning lines and a plurality of signal lines to thereby function as a shutter for adjusting light emitted from a light source such as a backlight. The liquid crystal display apparatus of this type has an advantage that it can display images with higher definition than a display apparatus such as a CRT (cathode ray tube), and is widely used for televisions, computers, information portable or mobile terminals, and the like.
However, as the definition or resolution of images increases, it becomes increasingly difficult to allow the scanning lines, the signal lines, and the auxiliary capacitor electrode to have a fine structure respectively. Thus there will rise a problem that a light transmission area, namely, an aperture ratio decreases, which results in reduction of brightness. To solve the problem, various researches for increasing the aperture ratio have been made.
A liquid crystal display apparatus having an increased aperture ratio is disclosed in Japanese Patent Application Laid-Open No. 11-311805. The liquid crystal display apparatus has a construction in which an insulation film is sandwiched between two transparent conductive films to form an auxiliary capacitor. In the construction of the liquid crystal display apparatus, the opening through which light passes is not affected by the area of the auxiliary capacitor. Thus the auxiliary capacitance is sufficiently secured and at the same time, the aperture ratio is increased.
FIG. 9 is plan view showing the construction of the above liquid crystal display apparatus. FIG. 10 is a sectional view taken along line X—X of FIG. 9. The construction of the liquid crystal display apparatus will be described below with reference FIGS. 9 and 10 in which only one pixel is shown for the sake of simplicity although there are multiplicity of pixels.
As shown in FIGS. 9 and 10, in a matrix board 51, a plurality of scanning lines 53 (only two of which are shown) and a plurality of signal lines 54 (only two of which are shown) are disposed on a glass substrate 52, and TFTs (thin film transistors) 55 (only one of which is shown) are formed at the intersection points of the scanning lines 53 and the signal lines 54. A planarization film 56 is formed in such a way as to cover the entire screen. Auxiliary capacitor electrodes 57, insulation films 58, and pixel electrodes 59 are sequentially formed over the planarization film 56. The planarization film 56, the auxiliary capacitor electrode 57, the insulation film 58, and the pixel electrode 59 are made of transparent materials respectively. Each pixel electrode 59 is connected to a drain electrode 61 through a through-hole 60. The auxiliary capacitor electrode 57 and the insulation film 58 have partially been removed in a predetermined pattern (shown with reference numeral 62 in FIG. 9) at the through-hole 60.
The planarization film 56 serves to improve the flatness of the pixel electrodes 59. In addition, the planarization film 56 is capable of insulating the signal lines 54 from the auxiliary capacitor electrodes 57. Therefore, the auxiliary capacitor electrode 57 can be formed all over the screen.
To prevent optical leak from the periphery of the pixel electrode 59, an opposed board 64 sandwiching a liquid crystal layer 63 between itself and the matrix board 51 has a light-shielding film 66 and an opposed electrode 67 made of a transparent conductive film on a glass substrate 65.
Because a transparent conductive film is used for the auxiliary capacitor electrode 57, the pixel electrode 59 other than a part shielded from light by the scanning line 53, the signal line 54, and the light-shielding film 66 serves as the opening of the pixel, and the auxiliary capacitor is formed to have an area almost the same as the area of the pixel electrode 59.
The above design of the liquid crystal display apparatus is intended to obtain a sufficient auxiliary capacitance and a high aperture ratio at the same time. However, the design invites the orientation disorder of the liquid crystal in the periphery of the pixel electrode, which causes conspicuous deterioration of the display quality.
FIG. 11 is a sectional view taken along line XI—XI of FIG. 9. The reason for the conspicuous deterioration of the display quality will be described below with reference to FIG. 11.
As shown in FIG. 11, an orientation film (not shown) is applied onto the inner side of each of the matrix board 51 and the opposed board 64. By carrying out rubbing treatment on the orientation film, liquid crystal molecules 68 of the liquid crystal layer 63 are oriented in almost parallel with each other and tilted at an equal angle. This angle is called a pre-tilt angle 69, at which the liquid crystal molecules are raised on a downstream side of the rubbing direction 70.
When the liquid crystal display apparatus displays an image, a voltage corresponding to a video signal is applied to the pixel electrodes 59. As a result, the liquid crystal molecules 68 are oriented in a direction along lines of electric force according to the pre-tilt angle 69, and an optical modulation is made according to a change of the orientation state thereof. To prevent a potential difference from being generated between the auxiliary capacitor electrode 57 and the opposed electrode 67, a voltage substantially equal to that applied to the opposed electrode 67 is applied to the auxiliary capacitor electrode 57.
At this time, an electric field is generated almost perpendicularly at the center 71 of the pixel electrode 59. Thus the orientations of the liquid crystal molecules 68 change uniformly at the center 71 of the pixel electrode 59. However, in the periphery of the pixel electrode 59, a strong transverse electric field (in FIG. 11, the direction of the electric field is shown with broken lines) is generated from the pixel electrode 59 to the auxiliary capacitor electrode 57. Therefore the orientations of the liquid crystal molecules 68 become non-uniform under the influence of the electric field. In an extreme case, when a voltage is applied, the rotational direction of the liquid crystal molecules 68 in the periphery 72 of the pixel electrode 59 becomes opposite to that of the liquid crystal molecules 68 at the center 71 of the pixel electrode 59 to generate a phenomenon called a “reverse tilt” (in FIG. 11, an estimated or presumed rotational direction of liquid crystal molecules is shown with arrows). In such a case, the orientation disorder reaches as far as an internal portion of the pixel electrode 59. Consequently optical leak is generated in the periphery of the pixel electrode 59, which deteriorates the display quality of the image greatly.
To prevent the influence of the orientation disorder, it is conceivable to light-shield the region in which the orientation disorder is generated, by increasing the width of the scanning line, the signal line, and the light-shielding film. However, such a method results in decrease of the aperture ratio. On the other hand, increase of the thickness of the auxiliary capacitor insulation film allows decrease of the influence of the electric field from the auxiliary capacitor electrode, but leads to decrease of the auxiliary capacitance. Therefore the auxiliary capacitance cannot be secured sufficiently.